As a measuring device functioning to measure a wear of a switching contact, being one of state quantities of a breaker, for example, there is the one in which an indicator is attached to a driving rod adjacent to a driving coil of an electromagnetic operating mechanism, a position thereof is detected using an optical detector, and a displacement of the indicator from at an initial position due to the wear of a contact (refer to, for example, Patent Document 1).
Patent Document 1: British Patent Application laid-open under Publication No. 2350724 (15-20 lines on page 5, and FIG. 4)